Bandgap generator circuitry is well known in the art of semiconductor integrated circuits, and examples of known bandgap generators 10a and 10b are shown in FIGS. 1A and 1B respectively. While it is not important to explain the intricate details of the operation of such well-known bandgap generator circuits 10a, 10b, it is noted that the point of such circuits is generally to provide a stable reference voltage, Vbg, to the integrated circuit in which the bandgap generator is located. Specifically, it is important that the reference voltage, Vbg, be (relatively) insensitive to temperature. For reasons well known to those skilled in the art, Vbg is so-named because it essentially equals the value of the bandgap of intrinsic silicon (1.2 eV) as scaled to volts from the Coulombic level (i.e., 1.2 V).
Bandgap generators usually incorporate elements with known temperature sensitivities in the hopes of “cancelling out” such sensitivities in the to-be-generated reference voltage, Vbg. Thus, in both of the exemplary bandgap generators 10a, 10b of FIGS. 1A, 1B, diodes are used. As one skilled in the art will recognize, such diodes can be traditional P-N junctions (e.g., such as D1 and D2), or can comprise P-N junctions in a bipolar transistor. For example, NPN transistor 11 in FIG. 1B is wired as a diode by virtue of the coupling of its base and collector nodes. For more information concerning bandgap generators, the reader is referred to Johns & Martin, “Analog Integrated Circuit Design,” Wiley and Sons, pp. 354-55, 360-61 (1997), which is incorporated herein by reference.
Regardless, diodes have a known temperature dependence. More specifically, the voltage across the diode, VD1, is essentially about 0.6 V at a nominal temperature (e.g., 50 degrees Celsius), and varies by about −2 mV/C (i.e., dVD1/dT=−0.002). Accordingly, the voltage across the diode, VD1, is approximately 0.5 V at 0 degrees Celsius, and is approximately 0.7 V at 100 degrees Celsius. The temperature dependence of the diode voltage, VD1, is illustrated in FIG. 3B.
Again, while not worth explaining in its exhaustive detail, the bandgap generator 10a, 10b, generates a reference voltage, Vbg, which is temperature independent, which is very useful on an integrated circuit. For example, in a dynamic random access memory (DRAM) integrated circuit, a stable non-temperature-varying reference voltage, Vbg, or derivative thereof (DVC2), can be used in the sensing of the charges stored on the memory cells of the array. Because such cells generally store charges equivalent to the power supply voltage (Vcc) (logic ‘1’) or ground (GND) (logic ‘0’), a voltage between these two (Vcc/2 or DVC2) is used as the comparison for sensing. Because this sensing reference voltage should not vary with temperature, it is preferably generated using Vbg. This is illustrated simply in FIG. 2, which shows a DRAM integrated circuit 20 having a bandgap generator 10a or 10b, which produces Vbg and feeds the same to a generator 25 to produce the sensing reference voltage of DVC2.
The use of a temperature-stable reference voltage Vbg for the purpose of producing the sensing reference voltage in a DRAM is but one example of the utility of a bandgap reference voltage, Vbg. Many other types of integrated circuits employ bandgap generators to produce temperature-stable reference voltages for a whole host of reasons.
Also common to integrated circuits are temperature sensors for monitoring the ambient and/or operating temperature of the integrated circuit in which the temperature sensor is located. Generally, temperature sensors, like bandgap generators 10, contain temperature-sensitive elements. However, in a temperature sensor, the temperature sensitivity of the elements are specifically exploited to produce a temperature-sensitive output, in stark contrast to a bandgap generator in which the temperature-sensitive elements are used to cancel temperature effects in the output. The output of a temperature sensor may be analog in nature, i.e., may produce a voltage or current whose magnitude scales smoothly with the sensed temperature, even if that value is digitized by an analog-to-digital (A/D) converter. Or, the output of a temperature sensor may be binary in nature. For example, depending on how the temperature sensor is tuned, it may produce a Hot/Cold* binary output signal that is logic high (logic ‘1’) when the temperature sensed is above a set point temperature, and is logic low (logic ‘0’) when below the set point temperature.
Temperature sensing can be performed in an integrated circuit for a number of reasons, but one important reason is to monitor power consumption in the integrated circuit. Generally, the more power (current) that is consumed by the integrated circuit, the hotter the circuit will become. At high temperatures, the integrated circuit may not perform well, or may even become damaged. Accordingly, temperature sensors can provide information to the integrated circuit regarding its temperature so that the integrated circuit can take appropriate corrective action, such as by reducing the operating frequency of the integrated circuit or disabling it temporarily to protect against thermal failure or damage. For example, in a DRAM, due to its volatile cell design, the contents of the memory cells must be periodically refreshed. However, due to increased current leakage at higher temperatures, refresh would need to occur more frequently at higher temperatures. But increasing the refresh rate will in turn increase power consumption in the integrated circuit, and will further increase its temperature, hence necessitating even more frequent refresh, etc. In short, a runaway condition can occur in which the temperature of the DRAM escalates. Eventually, the temperature of the DRAM may become sufficiently high that the DRAM could latch up, or become permanently damaged. Thus, a temperature sensor could provide the integrated circuit important information to ward off such potential operational problems.
Because or their utilities, both bandgap generators and temperature sensors are often used on the same integrated circuit. This is illustrated in simple form in FIG. 2, which shows a block diagram of an integrated circuit 20 having a bandgap generator 10a, 10b for producing a temperature-insensitive reference voltage, Vbg, as well as a temperature sensor 27 for producing a binary output (Hot/Cold*) indicative of the temperature of the integrated circuit 20 versus some temperature set point.
While both bandgap generators 10 and temperature sensors 27 are useful, it is unfortunate that they both independently take up significant real estate on the integrated circuit 20. However, because these circuits differ with regard to the temperature dependence of their output signals (the output signal of the bandgap generator is specifically designed to be insensitive to temperature whereas the output signal of the temperature sensor is specifically designed to be sensitive to temperature), it is believed that those of ordinary skill in the art have seen no logic to combine them in an effort to preserve valuable integrated circuit real estate. As will be seen in the description that follows, presented herein is an effective combination of a bandgap generator and a temperature sensor which is easy to implement, which takes up a smaller amount of real estate than the combination of both circuits taken individually, and which can be trimmed to provide a set point temperature suitable for the application at hand.